An RMMplex is one or more z/OS systems each running a DFSMSrmm subsystem sharing a control data set. An RMMplex can optionally include one or more DFSMSrmm subsystems as servers, one or more client subsystems, in addition to standard DFSMSrmm subsystems. The server subsystems and standard subsystems have direct access to and share the DFSMSrmm control data set. The client systems have no direct access to the DFSMSrmm control data set, but share the control data set through the server. All systems that share a control data set in this way are part of the same RMMplex.